\documentclass[11pt,a4paper]{article}
\usepackage{longtable}
\usepackage{arydshln}

\begin{document}

\renewcommand{\arraystretch}{2.3}
\begin{longtable}{l:l}
\textbf{Amplitude} & \textbf{Amplitude*} \\ \hline
$\displaystyle \frac{i}{p^2 + i\epsilon}$ & 
$\qquad \displaystyle -\frac{i}{p^2 - i\epsilon}$ \\
$\displaystyle \frac{i}{p^2 + i\epsilon}$ & 
$\qquad \displaystyle -\frac{i}{p^2 - i\epsilon}$ \\
\multicolumn{2}{c}{$something$} \\
\multicolumn{2}{c}{$something$} \\
$-ie \gamma^\mu$ & $\qquad ie \gamma^\mu$ \\
$-ie \gamma^\mu$ & $\qquad ie \gamma^\mu$ \\
\end{longtable}

\end{document}
